Jingwei Xie

and 2 more

We propose a multifaceted isoneutral eddy transport diagnostic framework that combines the stationary-transient and Leonard’s decomposition in large eddy simulation (LES). We diagnose the subfilter flux, the isotropic transport coefficient, and the anisotropic transport tensor or eigenvalues in the Southern Ocean (SO). The anisotropic tensor greatly reduces the reconstruction error of the subfilter flux because of its ability to distinguish the directionality of dynamic information, especially the topographic effect. A thorough analysis of the anisotropic tensor or transport eigenvalues reveals that the sign combination of the transport eigenvalues of the symmetric tensor links to the evolution of domainintegral large-scale PV enstrophy and the combination of different signs is most often, meaning the dominance of filamentation process in the SO. In the region with intense anisotropy, the dominant eigenvector tends to be perpendicular to the large-scale PV gradient, indicating an important role of the PV barrier mechanism in the SO transport process. The two distinct decompositions leveraged in our framework generate intriguing and profound results. Under the stationary-transient decomposition, we find a significant stationary contribution and the duality of the topographic effect which can not only anchors stationary structures but also organizes transient motions. Leonard’s decomposition, allows us to investigate the collective effects of the standing wave train, cross-scale interaction, and subfilter eddy-eddy interaction on the filtered space-time scale. We emphasize the complete subgrid flux, not the mere Reynolds term, and the LES framework needs to be considered in the subgrid parameterization of the coarse resolution ocean model.

Jingwei Xie

and 2 more

Mesoscale ocean eddies dramatically impact oceanic material transport, momentum and energy budgets, and large-scale ocean circulation; therefore, reasonably diagnosing their effects is crucial for providing insights into eddy parameterization scheme development. In this work, a Reynolds and coarse-graining hybrid eddy transport diagnostic framework is proposed and applied in the Southern Ocean. Both the isotropic transport coefficient and anisotropic transport tensor are diagnosed and decomposed into contributions from transient and stationary eddies. The tensor can be split into its symmetric and antisymmetric parts, and the symmetric tensor is further diagonalized to analyze the eigenvalues and eigenvectors. We verify that the anisotropic assumption better fits the ocean mesoscale eddy transport process than the isotropic assumption, at least in the Southern Ocean. We place particular emphasis on the transport tensor’s stationary component affected by large-scale topographies, nonconservative processes, and large-scale flow structures and find that its influence is highly anisotropic horizontally and varies vertically. We probe all tensor-related elements that emerge in our hybrid framework, especially the eigenvalues and eigenvectors of the symmetric tensor. We reveal all three configurations of the major and minor eigenvalues that appear in the Southern Ocean, where the one representing vortex filamentation is the most common scenario. In addition, we discover a high randomness of the eigenvectors, which implies the possibility of a semideterministic and semistochastic anisotropic parameterization scheme.

Jinbo Xie

and 7 more

A reasonable representation of orographic anisotropy in earth system models is vital for improving weather and climate modeling. In this study, we implemented the orographic drag scheme, including 3-D orographic anisotropy (3D-AFD), into the Chinese Academy of Sciences Earth System Model version 2 (CAS-ESM 2.0). Three groups of simulations named sensitivity run, medium-range forecast, and seasonal forecast respectively were conducted using the updated CAS-ESM model together with the original 2-D isotropic scheme (2-D) and the 3-D orographic anisotropy for the eight-direction scheme (3D-8x) to validate its performance. Sensitivity runs indicated that the simulated drag using the original 2-D scheme did not change with the wind directions, while the simulated drag using the updated 3D-AFD showed a smoother transition than that using 3D-8x. The 3D-AFD and 3D-8x had also about 80% larger drag and smaller wind speed of 1m/s than that of the 2-D scheme. Enhanced drag in the medium range and seasonal forecast using the updated CAS-ESM both alleviated the bias of the overestimated wind speed and the cold bias over mountain regions in the 2-D scheme. This was more apparent in winter (0.4-0.5 m/s and ~1K) than that in summer (0.1 m/s and ~0.1K) for the northern hemisphere region, such as the Tibetan Plateau. The vertical wind profile was also improved in the seasonal forecast. The results suggested that a reasonable representation of the orographic anisotropy was important in climate modeling, and the updated model of CAS-ESM with 3D-AFD alleviated the bias of the mountain wind.